My invention relates to a method for extracting metals from scrap batteries. An object of my invention is to bring about dissolution of the electrodes of electric storage batteries by electrolysis in an electrolytic solution. A further object is dissolution of the electrodes of electric storage batteries by means of electrolysis in an electrolytic solution with deposition of the metals on cathodes external to the batteries. Another object is to produce the electrochemical forced dissolution of the electrodes above mentioned while immersed in an electrolyte by means of a power supply, with the deposition of the metals on cathodes external to the batteries. Also another object is to produce, by dissolution of spent electric storage batteries, oxides and suboxides on anodes immersed in the same electrolyte.
In accordance with my invention, a complete electrochemical system, the spent battery, is used as part of another larger electrochemical system. In my process four different electrode reactions are taking place at the same time, their rate also being in steady-state equilibrium with one chemical reaction occurring in the electrolyte. The process of my invention is suited for extraction of metals from all secondary electric storage batteries.
For the purpose of illustrating my process, I will use as a practical example the lead-acid electric storage battery. The end of the useful life of these batteries is determined by the accumulation on the electrode surfaces of lead sulfate crystallized in the form known as hard sulfate. These crystals, unlike the finely dispersed sulfate which constitutes the product of normal battery discharge, resist anodic reoxidation to dioxide during recharging. Since lead sulfate is an electrical insulator, battery capacity is reduced proportionally to the area of the electrodes covered with crystals. Since the quantity of lead contained in batteries that is actually used electrochemically is in the range of 25% - 50%, most spent storage batteries classified as scrap consist of lead sulfate, lead dioxide, up to 12% of antimony, tin or calcium alloys and metallic lead.
In accordance with this invention, it has been found that antimony, tin, calcium and titanium are left behind in the treated battery containers in the form of anodic mud or slimes, especially when an external power source is used and when external electrodes are used for collection of lead and lead suboxide. It has further been found that recovery of anodic slimes containing grid metals and grid alloying metals is particularly effective if the electrolyte used during the dissolution process is based on sulfamic acid and if the recovery of lead is carried out at a temperature near room temperature.
Also, some of the elements released by the dissolving battery electrodes and connections into the electrolyte solution, such as iron, arsenic, and antimony, can be removed from the electrolyte by blowing air through the solution, to thus obtain a selective oxidation of these impurities. The oxygen in the air reacts with elements in the solution less noble (more active) than lead to form compounds with limited solubility which precipitate as a sludge. It is generally difficult to determine what compounds are formed, since the sludge also contains the species forming the electrolyte; as an example, the sludge can be a mixture of oxides, hydroxides, basic oxide, and/or hydrated oxy-compounds of the anion of the electrolyte with the metal.
This air can be blown through the electrolyte in the main tank or separately in a purifying column through which the electrolyte is circulated and through the bottom of which air under pressure is blown through perforated pipes or plates. Where the electrolyte temperature is maintained above the ambient air temperature, the air can be heated to maintain the electrolyte at its operating temperature.
Traditionally, recovery of lead from spent batteries requires physical demolition of the batteries by crushing, shredding, and classifying steps in which significant amounts of lead are lost. Besides, these operations create large volumes of battery-container fragments which are difficult to dispose of. The scrap thus segregated has heretofore been refined using pyrometallurgical processes, which generate large quantities of fumes from the combustion of fragments of the battery containers and separators still attached to the lead scrap. The worst aspect, however, is the sulfur dioxide emission from lead sulfate decomposition.
Presently, air quality specifications, fuel costs, and the costs of battery crushing and scrap classification, do not permit economically feasible extraction of lead values by pyrometallurgical methods.
In addition, improved storage battery construction will tend to worsen the problems of recovering lead from spent batteries, which will have thinner and more numerous electrodes for increasing battery efficieny; require complex container shapes which are more difficult to demolish; have grids made of metals such as titanium to decrease battery weight; and which further require a separate process for recovering these valuable metals.